[coRtALoS]

Honestly the only other thing I can think of for something like that is malware, otherwise you've got me completely baffled. I could jump on the "it's something wrong with your ISP" bandwagon, but they'll most likely say if it's only happening with Starcraft, than it's an issue with Blizzard games.

One main point I didn't mention is whether you're running any third party program while you're playing Starcraft. Are you using stuff like Oblivion or PenguinPlug? Or have you uninstalled it since your most recent reformat? Might not be completely off your system.